I've watched your video on how to remove the back, but I can't get it off.

To reset contrast on the sunnuto t3d hold down all 4 corner buttons at once, this will place the watch in sleep mode. Wait a few seconds and press any button to wake the watch back up. This will allow you to run through the inital set up just like when you got your watch, alot of your info will be saved so you can just quickly confirm them with the mode button. I also set my contrast too high, I have reset it to 3 and found it much easier to read in all lighting.

To reset the watch, you need to 'short' the reset point with the battery. To do this, with the watch face down and the back removed the 'Reset' point is in the bottom right hand corner (a gold-coloured contact point). Use a pair of metal tweezers or an unbent paper-clip (anything as long as it's metal and will touch the reset point and the back of the battery at the same time). Maintain contact for about 5 seconds and you should get the display back.
I've just changed the battery in my SDB 500W and the above method worked. I did lose the audible 'beep' though and I'm currently on the net trying to find how to get it back. That's how I found your question... :)

Definitely yes, unless you've got some cheap copy.

The vast majority of Fossil analog wristwatches take either a #377 or #379 watch battery. The #379, being slightly smaller, is somewhat more commonly found in women's watches than in men's watches, but I have seen both types used in men's and women's watches alike.
These batteries are commonly found at many drug stores, jewelry store counters at places like Walmart and Target, and even at some dollar stores. Be aware that cheap dollar store batteries are usually alkaline, not silver oxide, versions of the same size battery. Alkaline batteries may work perfectly well in many watches, but they have a somewhat different energy performance curve over time, and, in general, they won't last quite as long as a comparable silver oxide battery. In addition, for reasons I've never been able to figure out, some of the Fossil watches that I've serviced would not function with an alkaline battery--but would work fine when I put in a silver oxide battery of the same size and voltage. I have not been able to see a pattern to predict when this will and will not occur.
As a final thought, the backs of some Fossil watches are very snugly fitted to their cases. You can generally get them off without too much of a problem, but there's a good chance you may need a jeweler's press to get them to snap back on properly. I've noticed this most with round watch backs; I generally haven't needed a press to close oblong or tonneau-shaped Fossil watches (or some round watches, too). A jeweler's press spreads the pressure evenly around the edges of the watch back and watch case, preventing damaging pressure on the watch crystal, watch movement, and watch back. Clamping the watch in a regular vise to try to press on the back runs a high risk of damaging your watch, and I do not recommend trying that, no matter how frustrated you get. It's much safer (and cheaper, overall) to tip someone with a vise a couple of dollars to close up your watch for you.

mcdevito75 Here, You"ll need a special toll called a link remover tool, available on ebay, however if you"ve never done work or this particular work on watches, your best bet is too find a small watch repair shop in your area for the link/s removal.

mcdevito75 here, Even with the proper tools we still have to improvise at times, Instead of useing your case opener from your tool kit, use a steak knife or similar with a sharp edge inside that groove you"ve described, the cover will pry off.

this video will show you were the spring goes next to battery

Probably bad contact on battery, try removing back tweaking battery and then short AC or AR to +ve before reaasembly